One of the more prevalent type of freezers used to provide cryogenic freezing of a product such as a foodstuff is a continuous, in-line tunnel that utilizes liquid nitrogen as an expendable refrigerant. One such apparatus in commercial use is shown in U.S. Pat. Nos. 3,813,895 and 3,892,104, the specifications of both patents being incorporated herein by reference. Freezers of this type sold by Air Products and Chemicals, Inc. under the trademark CRYO-QUICK are continuous, in-line tunnels that utilize liquid nitrogen as the expendable refrigerant. Freezers of this type can achieve high thermal efficiency because they are designed as counterflow heat exchangers. In these freezers liquid nitrogen is sprayed directly onto the food product near the discharge end of the freezer or tunnel. As the liquid nitrogen (LIN) is vaporized, a very cold gas at temperatures of -320.degree. F. (-196.degree. C.) is formed. This cold nitrogen gas is then caused to move through multiple zones of gas recirculation as it flows towards the entrance end of the freezer. Since the maximum available refrigeration of the LIN has been utilized at that point, the warm nitrogen gas can be vented to the atmosphere by an exhaust system.
Liquid nitrogen that is an equilibrium at 35 psia (241 kPa), has a latent heat of 80.5 BTU/lb (187 J/g) when it is vaporized at atmospheric pressure. Gaseous nitrogen has a sensible heat of 79.5 BTU/lb (185 J/g) when it is warmed from -325.degree. F. (-196.degree. C.) to 0.degree. F. (-18.degree. C.). Thus, liquid nitrogen has a total available refrigeration of 160 BTU/lb (372 J/g) under these conditions. Since the sensible heat of the gaseous nitrogen is almost one half of the total available refrigeration, it is necessary to provide effective convective heat transfer to achieve high thermal efficiency in the freezer.
When all other conditions are constant, the forced convection heat transfer coefficient varies as the gas velocity raised to the 0.8 power. As a result, if the gas velocity is doubled, the forced convection heat transfer coefficient will increase by only a factor of 1.74. Further, the volume of gas delivered by a centrifugal fan varies with the fan speed, but the fan power varies with a cube of fan speed. Thus, if the fan speed is doubled to produce greater gas velocity, the fan power will increase by eight times. However, the fan power is an energy input into the freezer and must be minimized to achieve high thermal efficiency. Thus, the most efficient liquid nitrogen food freezer requires a recirculation system that can achieve a high forced convection heat transfer coefficient with the lowest fan energy input.
Another problem with a cryogenic freezer is the presence of frost caused by wet food products, such as raw shrimp, or by a humid operating environment. These circumstances may cause frost to accumulate on the recirculating fans resulting in a significant decrease in the gas velocity at the surface of the food product. For example, a conventional forward curved centrifugal fan will become clogged with frost in less than one hour, virtually eliminating all gas recirculation. Further, the decreased pressure at the fan inlet will promote the formation of frost, tending to clog the inlet to the recirculating fan.
There are several known solutions to the problem of providing gas recirculation within a cryogenic freezer. One type is an axial fan blade that moves gas in the direction of the axis of fan rotation. Axial fans, sometimes called propeller fans, have several disadvantages in a cryogenic freezer when located above the conveyor belt that moves the product from an inlet end to the exit end of the freezing tunnel, thus directing gas downwardly to the conveyor belt. The gas velocity at the conveyor belt covers a circular area only slightly larger than the fan diameter. Further, the gas velocity is non-uniform across the width of the conveyor belt resulting in non-uniform freezing of the food product. However, the most serious problem with an axial fan blade is the effect of frost formation. Frost will accumulate on and above the fan blade to substantially reduce the gas velocity. Furthermore, the frost deposited on the fan blade will cause vibration problems leading to early mechanical failure of the blades and fan shaft bearings.
U.S. Pat. No. 4,276,753 is typical of a freezer employing the propeller type circulating fans.
The current CRYO-QUICK freezers use a radial type fan with blades that move gas radially outward from the axis of fan rotation. These blades are shown in the '895 patent, most clearly in FIG. 4. As shown in the '895 and '104 patents, as well as the U.S. Pat. No. 4,800,728 and U.S. Pat. No. 4,475,351 the fan blades are positioned just below the inside surface of the roof of the freezing tunnel. The gas recirculation path is radially outward from the blades, down onto the conveyor belt and upward into the fan. The primary benefit of the radial fan blades is its tolerance to frost formation. A significant frost layer, e.g. approximately one-half inch (13 mm) thick does not significantly impair gas recirculation. Furthermore, a frost layer results in fewer vibration problems because the fan blade operates at a lower speed (e.g. 1140 rpm) driven by a shorter fan shaft than an axial fan blade. The radial fan blade provides reasonably uniform gas velocity at the surface of the product being frozen, but has a limitation caused by the recirculation path. Since the gas is lifted upward from the conveyor belt, light products (e.g. food) may be lifted or disturbed by the recirculating fan.
Another type of recirculating fan is shown in U.S. Pat. No. 4,589,264. In the '264 patent the recirculating fans employ a wheel that has four flat paddles placed immediately above the food product. Patentee stresses that the "gas area adjacent the fan wheel is fluctuating and turbulent . . ." Although patentee in FIG. 3 shows gas spiralling downward toward the hub of the fan, the negative pressure at the center of the fan will pull gas equally from above and below the fan wheel. As a result, the area inside the paddles or fan wheels will pull gas upwardly from the conveyor belt and lift or disturb light products being frozen. Furthermore, while patentee indicates that the hub of the fan wheel can be displaced on the shaft in order to raise or lower it, patentees FIG. 3 clearly shows that the shaft of the fan will interfere with the food product if the fan wheel is raised to accommodate a product having greater height.